1. Field of the Invention
The present invention relates to a Raman amplifier of a wavelength division multiplexing (WDM) optical transmission system.
2. Description of the Related Art
In large-capacity WDM optical transmission systems, erbium-doped fiber amplifiers (EDFA) and Raman amplifiers are used to obtain a broad gain bandwidth and a small noise figure.
The Raman amplifiers couple outputs from pump lasers of several different wavelengths using WDM optical couplers, send the coupled outputs from the pump lasers to optical fibers that is transmission paths, and amplify signal light.
Raman amplifiers use a plurality of pump lasers of different wavelengths to get good Raman gain flatness in a signal bandwidth. However, since pump lasers of different wavelengths generate Raman interaction with one another, the pump laser of the shortest wavelength requires an output much higher than the other pump lasers. Also, four-wave mixing may occur among the pump lasers wavelengths in a specific other transmission fiber type.
C. R. S. Fludger suggests a time division multiplexing (TDM) method in his paper “Novel Ultra-broadband High Performance Distributed Raman Amplifier Employing Pump Modulation” (OFC2002, WB4, pp. 183–184 (2002)). In the TDM method, pump lasers of different wavelengths are sequentially turned on and off over time in turn. Therefore, the pump lasers of difference wavelengths do not overlap in the fiber. The TDM method eliminates Raman interactions among pump lasers of different wavelengths, but requires many pump lasers of different wavelengths like conventional Raman amplifiers.
L. F. Mollenauer demonstrates in his paper “Time-Division Multiplexing of Pump Wavelengths to Achieve Ultra-broadband, Flat, Backward-Pumped Raman Gain” (Optics Letters, vol 27, pp. 595 (2002)) that it is possible to effectively design a Raman amplifier having a broad gain bandwidth and superior gain flatness using a single pump laser that has the features of a changeable and tunable wavelength.
When wavelengths of pump lasers change, contribution to Raman gain at each of the wavelengths is controlled by changing the duration of each of the wavelengths. In the paper, outputs from pump lasers are assumed to remain constant when the wavelengths of the pump lasers are changed. However, in practice, there is no pump laser whose output remains constant when the wavelength of the pump laser changes. Also, since a change in wavelength affects laser dynamics, outputs from pump lasers become unstable or transient phenomena occur when the wavelengths of the pump lasers change.